A plain bearing is a bearing in which the body which is rotating and is to be borne slides on sliding surfaces. In this case, the sliding surface of the body to be borne slides on the sliding surfaces of the bearing body. Depending on the nature of the plain bearing, the bearing body is designed differently. In the case of a radial plain bearing, the bearing body is in the form of a bearing shell which is embedded in a housing and may be subdivided into a plurality of bearing segments.
During operation of the plain bearing, the sliding surface of the body to be borne rubs on the sliding surfaces of the bearing body. In order to keep this friction, and therefore the wear of the plain bearing as well, as low as possible, a lubricant film, for example an oil film, is provided between the sliding surfaces.
When rotation starts, the lubricant is drawn into the gap between the sliding surfaces, and the body to be borne moves to an eccentric position in the bearing body. During this process, it passes through the range of mixed friction. As the sliding speed and the movement into the eccentric position increase, a lubricant wedge is formed between the body to be borne and the bearing body, whose pressure results in the body to be borne being lifted off the bearing body. The body to be borne then runs in a stable form in the bearing body, in the case of purely hydrodynamic lubrication without any direct contact between the bearing body and the body to be borne. This effect occurs in a radial plain bearing by a bearing clearance that is provided.
A plain bearing, in particular a radial plain bearing of the type mentioned initially, can be used, for example, in steam-turbine, generator and gas-turbine construction. In this case, the plain bearing bears rotor shafts as the bodies to borne.
The objective of current new developments in steam-turbine construction for electricity generation is to significantly increase the efficiency and to convert the energy efficiently. In this case, high operational reliability must be ensured, and cost savings must be achieved. As a result of this development:    a) the rotors are becoming longer, and the bearing separation is becoming greater,    b) the evaporation cross sections are becoming greater, particularly in the low-pressure area, and    c) the specific power of the individual turbine elements is increasing.
While at the same time maintaining a so-called mono block construction principle, particularly in the low-pressure area of steam turbines, this development is in the end leading to a significant increase in the specific bearing loads. The bearings and their maximum load-bearing capacity are therefore becoming limiting factors for turbine development, particularly in the low-pressure area. A similar development can also be observed in the case of generators and gas turbines. The load-bearing capacity of the plain bearings is limited by the maximum temperature, the maximum specific bearing load and the minimum lubricating film thickness.
In addition to the significant increase in the load-bearing capacity, this bearing is subject to the following additional requirements:                high stability for all load ranges, and good rotor-dynamic characteristics,        maintenance friendliness,        very high operating availability/low-wear operation,        raising of the rotors by high-pressure oil during rotation        high reliability (integrity of the turbine set with high unbalance levels),        use in the existing bearing housing, and        retrofit capability.        
In order to comply with the abovementioned requirements and in order to significantly increase the load-bearing capacity of the plain bearings, the following measures, for example, are known:                reduction of the rotor weight,        use of high-viscosity oils,        greater bearing widths and/or bearing journal diameters,        lower oil supply temperature, and        directional (directed) lubrication (in the case of tilting pad bearings).        
These standard measures to increase the load-bearing capacity have been proven in practice, but are reaching their limits ever more frequently.